setting culvert question

[da'yoop]

When placing culvert, do I need to 'bed' the pipe with any particular material? I'm trying to think ahead so it doesn't wash out under or around it. This is a slow moving stream about a foot deep and maybe 2feet wide.

 

[mitch504]

If that stream doesn't ever really flood, I usually just use the natural clay. If I'm worried about it, I put some large, 4-6" rock under the end at the upstream end.

 

[Haddy]

Depends on material that is there now but I normally dig out as much soft material as practical and replace it with 2 - 4 inch drainage metal , dont know what you call that but it's a clean metal , just the stones . I place it up the sides a little of the culvert as well

 

[da'yoop]

it's in soft, ground and it doesn't really flood..........maybe gets up to 2 feet in the springtime but that's about it. I can bed it and surround it with pit run if I need to. I plan on putting pit run over it anyway.

 

[stock]

What are the loads that are likely to cross the culvert? If the ground is soft under the pipe or box it may move when transversed by a load. You might need to place 12" lean-mix under the culvert if loaded lorries are crossing.
As I'm not from local I can advise you on the regulations you need to comply with. You may consider using a rip rap anti scarifying measures on the entrance and on the exit of the culvert or a combination of concrete and rip rap. Might also need to consider the habitat for the fish if there is any.

http://www.snh.org.uk/uplandpathwork/2.6.shtml

 

[CM1995]

When I am setting culverts I bed the bottom of the trench and when the pipe or box culvert is installed, up the the spring line with #57 stone. #57 stone is 7/8 to 1/2" stone, washed. The designation in our area for stone is the size screen it passes through so #57 would be 5,6,7 size stones.

If the culvert is just a bare pipe with no headwalls, I would place a good size rip-rap on both ends for wash protection and be generous with it. The remaining backfill, from the spring line to the desired grade can be whatever suitable material you have available to build your road with.

Backfill your crossing with your onsite suitable material and then place a nice layer of rip-rap around both ends.

On edit a second important thought - what size pipe are you using and is it enough to handle the flow? Most wash outs occur with an undersized pipe installed.

 

[da'yoop]

The only load would be my backhoe or possibly a trencher. The road will dead end within 75 feet after the culvert. I just need to get to the other side to do some work. No fish, it's an intermittent stream. Pretty much dries up in the summer, has maybe a 6 inch trickle. I just don't want it to wash out in the spring before I can get finished on the other side.

 

[stumpjumper83]

whatever you do, don't over dig. If its a 12" pipe, don't use a 48" bucket, as well as don't dig too deep, then pat it down and call it good... Headwalls are 3x as important as tail walls. If its more than a seasonal use road, to the top of pipe + with 57's and then I usually go to grade with 3's.

 

[da'yoop]

Seasonal only. It's a private crossing on my own property. I'm glad you said "don't dig too deep" I didn't really give that part much thought. Headwalls ok with 4"-8" rip-rap?

 

[stumpjumper83]

I usually use whatever locally occuring rock that I can find. headwall size is as big as I can carry myself, tail wall size is whatever stacks easily. For seasonal stuff, backfill with whatever you took out as long as it isnt sand, topsoil, or mud.

 

[DrJim]

The other members have provided excellent, sound advice. As CM1995 points out, one thing I see commonly overlooked is the pipe size. The maximum flow volume capacity is roughly proportional to the cross sectional area of the pipe; for round pipe, the area varies with the square of the radius. Think a 30" is "twice as big" as a 15"? Do the math. . . it's actually 4x "bigger" in terms of flow capacity.

I have a 24" (that I built a really, really nice "winged" headwall on) that is undersized--in heavy rainfall events water overflows the culvert and is eating the top and sides around and beyond the tailwall. A 36 would be too tall for the contours of the site, and my friends all rag me that " a 30" ain't enough bigger to help any." Again doing the math, comparing the radius squared of one to the radius squared of the other, the 30" is 225/144 the "size" of the 24", or 56.25% larger. It is indeed a significant difference, and I'll be tearing out my headwalls and pulling the 24" to replace with the 30". (Yes, the 30'' is more difficult to source.)

 

[DrJim]

The type of culvert also influences the flow capacity. For example, the double or triple-walled HDPE (plastic) culvert with a smooth interior has more flow-carrying capacity than corrugated galvanized metal, which in turn flows better than the more deeply-corragated non-lined HDPE plastic. The slope or “fall” matters, especially if the culvert/ditch has near-minimal fall, and the type of interior surface makes a huge difference in problems with sediment and trash accumulation in the pipe—the smooth HDPE plastic beats ‘em all in this regard.
As SJ 83 pointed out, the headwall is the more important than the tail. We have moved a lot of dirt on the farm, hauling maybe 800-1000 tri-axle loads of fill. Most of that I have spread with. . . my Bobcat 773, believe it or not. As the farm has absolutely no rock at all, I sift rocks of all sizes out of the fill material and stockpile the rock for use in building headwalls. Rather than dumping rip-rap and leaving large gaps and holes, I hand-stack the flatter rocks to build my headwalls, turning some longer rocks here and there lengthwise back into the wall, to keep the wall from toppling. The headwalls I stack are 2-3 ft thick.
I use thin layers of base stone to bed the hand-stacked stones, and for the top courses of stone, dump dry “Sakrete” into the base stone to secure the stones stacked above the diameter of the pipe.
And yes, I always lay a big flat rock, or a piece of scrap sidewalk flat in the bottom directly in front of the culvert inlet--it doesn't have to extend under the pipe.

 

[DrJim]

My 50 acres is all bottom land, and the mud under the culverts is soft. I don't find the loading (load bearing) of the bottom to be an issue for dozers, tractors, and an occasional concrete truck or tri-axle over the culverts. What would be an issue is the character of the sides of the ditch. Dig the ditch no wider than the conditions require--the solid walls of the ditch resist the tendancy of the pipe to deform. I use clay for my main backfill, and have some that are settled in nicely with the black bottom loam alone. Even with abundant limestone quarrying 5 miles from here, including the haul bill and sales tax, base stone costs $ 18/ton. I never use any purchased graded stone for backfill. I get enough dirt on top of the culverts immediately, to keep ‘em from floating out in next week’s flood, and fill the sides gradually, letting the rail settle and pack the dirt in alongside the culvert. Topping with the stone of your choice is a matter of your budget. Unless you have water flow over the culvert, compactable soil works fine for a utility road, and if you have water flow over. . . 2 or 3” stone won’t stay too long, either. Keep surface flows away from the culvert.

 

[da'yoop]

Thanks everybody for the help, I think I learn something every time I come to this forum. I have all the gravel I need about 100 yards away and it's got every thing from sand to bowling ball size rock.

 

[ScottAR]

If your really worried about the headwalls, 80 lb Sakcrete dry stacked running bond style with rebar staked through it makes a dandy short wall. The paper will rot off long after the sakrete has turned solid.

 

[Drc]

Pea Gravel and headwalls

 

[hackinjack]

This got no bedding

48in culverts 30 feet long before i rip-rap. Sides are a bit wide since i pushed a few 100 yds over it. But keep in mind just a few dump loads of gravel will go over it then just cars and atvs. I know its a bit big looking but it drains 600ish acres. The over flow swale is built in the the right. I will post pics when i got the rip-rap and gravel in place.

 

[da'yoop]

I see you have 2 culverts in place. I was going to use two 12" culverts, side by side. Or would I be better off using one 24" culvert?

 

[mitch504]

one 24" culvert flows the same as four 12" culverts, since it works on surface area.

any time you are dealing with round pipe, any size, it is roughly-"double the diameter, quadruple the flow"

 

[Haddy]

I think that you have good culvert size for the catchment area . I like to have the overflow area lower , away from the pipes and as wide as possable and compact s**t out of it